Nonvolatile semiconductor storage device and method of manufacture thereof

ABSTRACT

A nonvolatile semiconductor storage device including a number of memory cells formed on a semiconductor substrate, each of the memory cells has a tunnel insulating film, a charge storage layer, a block insulating film, and a gate electrode which are formed in sequence on the substrate. The gate electrode is structured such that at least first and second gate electrode layers are stacked. The dimension in the direction of gate length of the second gate electrode layer, which is formed on the first gate electrode layer, is smaller than the dimension in the direction of gate length of the first gate electrode layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2008-285516, filed Nov. 6, 2008,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonvolatile semiconductor storagedevice and more particularly to a nonvolatile semiconductor storagedevice in which the gate electrode structure of transistors that formmemory cells is improved and a method of manufacture thereof.

2. Description of the Related Art

Nonvolatile semiconductor memories which use metal oxide nitride oxidesemiconductor (MONOS) cells for memory cell transistors are structuredsuch that a tunnel insulating film, a charge storage layer, a blockinsulating film and a gate electrode are stacked on the device areas ofa semiconductor substrate. Data of the MONOS cell is erased by injectingholes from the semiconductor substrate through the tunnel insulatingfilm into the charge storage layer. At this point, electrons will beinjected from the gate electrode through the block insulating film intothe charge storage layer, causing a problem that sufficient erasingcannot be accomplished.

To solve the problem, conventionally, use has been made of a high-k filmwhich has a larger dielectric constant than the tunnel insulating filmas the block insulating film and a metal which has a larger workfunction than silicon as the gate electrode.

To prevent the short-channel effect of the memory cell transistors, itis required to increase the dimension in the direction of gate length ofthe gate electrodes. In addition, where a process in which the gateelectrodes are oxidized is included, it is required to further increasethe dimension in the direction of gate length of the gate electrodes.However, increasing the dimension in the direction of gate length of thegate electrodes becomes reducing space between adjacent gate electrodes.As a result of short-circuiting of adjacent gates is happen.

A proposal has been made for a device which is not the MONOS structurebut one of the nonvolatile semiconductor memories having a stacked gatestructure consisting of a floating gate and a control gate and which isformed such that the control gate is formed from two or more layers andthe top electrode layer is formed with a film of oxide on the side (forexample, Jpn. Pat. Appln. KOKAI Publication No. 2008-53295). With thisdevice, however, an oxide film (sidewall insulating film) is also formedon the floating gate side, offering a problem that the gate length isreduced correspondingly. Furthermore, it is required to form ananti-oxidation film between the two gate layers which serve as thecontrol gate, which makes the structure complex. Therefore, applicationof the structure disclosed in the publication to the MONOS structurefails to solve the aforementioned problems.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided anonvolatile semiconductor storage device including a number of memorycells, each of the memory cells comprising:

-   -   a semiconductor substrate;    -   a tunnel insulating film formed on the semiconductor substrate;    -   a charge storage layer consisting of an insulating film and        formed on the tunnel insulating film;    -   a block insulating film formed on the charge storage layer; and    -   a gate electrode formed on the block insulating film, the gate        electrode being structured such that at least first and second        gate electrode layers are stacked, the first gate electrode        layer is in contact with the block insulating film, the second        gate electrode layer is formed on the first gate electrode layer        and consists of a material different from that of the first gate        electrode layer, and the second gate electrode layer has a        smaller dimension in the direction of gate length than the first        gate electrode layer.

According to another aspect of the present invention, there is provideda nonvolatile semiconductor storage device including a number of memorycells, each of the memory cells comprising:

-   -   a semiconductor substrate;    -   a tunnel insulating film formed on the semiconductor substrate;    -   a charge storage layer consisting of an insulating film and        formed on the tunnel insulating film;    -   a block insulating film formed on the charge storage layer; and    -   a gate electrode formed on the block insulating film, the gate        electrode being structured such that first, second and third        gate electrode layers are stacked, the first gate electrode        layer consists of a metal material and is in contact with the        block insulating film, the second gate electrode layer consists        of polysilicon and is formed on the first gate electrode layer        so that its edges opposed to each other in the direction of gate        length retract relative to the corresponding edges of the first        gate electrode layer, and the third gate electrode layer        consists of a metal silicide and formed on the second gate        electrode layer.

According to a further aspect of the present invention, there isprovided a method of manufacturing a nonvolatile semiconductor storagedevice, comprising:

-   -   forming a tunnel insulating film on a semiconductor substrate;    -   forming a charge storage film of an insulating material on the        tunnel insulating film;    -   forming a block insulating film on the charge storage film;    -   forming a gate electrode on the block insulating film, the gate        electrode comprising at least first and second gate electrode        layers which are stacked and are different from each other in        material;    -   processing the second gate electrode into a gate pattern;    -   forming a sidewall insulating film on the sides of the second        gate electrode layer; and    -   etching the first gate electrode layer using the second gate        electrode layer and the sidewall insulating film as a mask.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a sectional view in the direction of gate length of a memorycell transistor portion of a NAND nonvolatile semiconductor memoryaccording to a first embodiment;

FIG. 2 is a sectional view in the direction of gate width of a memorycell transistor portion of the NAND nonvolatile semiconductor memoryaccording to the first embodiment;

FIG. 3 shows the circuit arrangement of a NAND nonvolatile semiconductormemory as an example of a nonvolatile semiconductor storage device usingmemory cells of the structure shown in FIGS. 1 and 2;

FIG. 4 is a schematic plan view of the NAND nonvolatile semiconductormemory of FIG. 3;

FIGS. 5A through 5F are sectional views, in the order of steps ofmanufacture, of the memory cell transistor portion in the firstembodiment;

FIG. 6 is a sectional view in the direction of gate length of a memorycell transistor portion of a NAND nonvolatile semiconductor memoryaccording to a second embodiment;

FIG. 7 is a sectional view in the direction of gate length of a memorycell transistor portion of a NAND nonvolatile semiconductor memoryaccording to a modification of the second embodiment; and

FIG. 8 is a sectional view in the direction of gate length of a memorycell transistor portion of a NAND nonvolatile semiconductor memory as acomparative example.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention will be described hereinafterwith reference to the accompanying drawings.

First Embodiment

FIGS. 1 and 2 are sectional views in the directions of gate length andgate width, respectively, of a memory cell transistor portion of a NANDnonvolatile semiconductor memory according to a first embodiment of thepresent invention.

Device isolation regions 14 is formed buried in a surface portion of asilicon substrate (semiconductor substrate) 11 so that device isolationregions 14 surround device regions. A tunnel insulating film 12 ofsilicon oxide is formed on the device regions surrounded by the deviceisolation regions 14. A charge storage layer 13 of silicon nitride isformed on the tunnel insulating film 12. Here, a surface of the deviceisolation regions 14 is substantially the same level as the chargestorage layer 13.

A block insulating film 15, which is an Al₂O₃ film for example, isformed on the charge storage layer 13 and the device isolation regions.A gate electrode 16 is formed on the block insulating film 15. Thetunnel insulating film 12, the charge storage layer 13, the blockinsulating film 15 and the gate electrode 16 are processed into a gatepattern as shown in FIG. 1.

The gate electrode 16 includes a first electrode layer 16-1 formed onthe block insulating film 15, a second electrode layer 16-2 formed onthe first electrode layer 16-1, and a third electrode layer 16-3 formedon the second electrode layer 16-2. For example, the first, second andthird electrode layers 16-1, 16-2 and 16-3 are made of TaN, polysilicon,and NiSi, respectively.

A first insulating film 17 of, say, silicon oxide is formed on portionsat both ends of the top surface of the first electrode layer 16-1 and onthe sidewalls of the second and third electrode layers 16-2 and 16-3 inthe direction of gate length. Source/drain diffusion layers 18 of amemory cell transistor are formed in the surface of the siliconsubstrate 11 so that the region between them is located below the gateelectrode 16. A second insulating film 19 as a silicon oxide is formedeach of the source/drain diffusion layers 18.

As a comparative example, the structure of a memory cell transistorportion of a conventional NAND nonvolatile semiconductor memory isillustrated in FIG. 8, in which corresponding parts to those in FIG. 1are denoted by like reference numbers. As shown in FIG. 8, first tothird electrode layers 16-1 to 16-3 are made substantially equal in thedimension in the direction of gate length. That is, in FIG. 8, thespacing between the second electrode layers 16-2 or the third electrodelayers 16-3 in the adjacent gate electrodes is shorter than in FIG. 1 bytwice the dimension of the first insulating film 17 in the direction ofgate length.

Furthermore, the upper portion of the third electrode layer 16-3 willexpand in the direction of gate length to be subjected to silicidation.As a result, the possibility will increase that the third gate electrodelayer 16-3 is shorted between the adjacent gate electrodes.

If the spacing of adjacent gate electrodes is increased in order toprevent the third gate electrode layer from being shorted between theadjacent gate electrodes in the structure of FIG. 8, the length of thegate electrode of the first to third electrode layers 16-1 to 16-3 willdecrease. Then the characteristic of memory cell transistors degrades.

Depending on the kind of metal used for the first gate electrode layer16-1, edges of the metal electrode may turn into an insulator afteroxidized by a thermal process. In such an event, the characteristics ofmemory cell transistors will degrade remarkably.

In the structure shown in FIG. 1, on the other hand, the first electrodelayer 16-1 has a larger dimension in the direction of gate length thanthe second and third electrode layers 16-2 and 16-3. Therefore, even ifthe upper portion of the third gate electrode layer 16-3 expands in thedirection of gate length, shortening of the second or third gateelectrode layer 16-2 or 16-3 between adjacent gate electrodes can besuppressed.

Moreover, the dimension in the direction of gate length of the firstgate electrode 16-1 does not decrease. As a result, the dimension in thedirection of gate length of the first electrode layer 16-1 whichelectrically functions as the gate length of a transistor can beincreased. For this reason, it becomes possible to prevent degrading thecharacteristics of the memory cell transistors due to the short-channeleffect.

As the block insulating film 15, use is made of a film of, say, Al₂O₃which has a larger dielectric constant than the tunnel insulating film12 of, say, silicon oxide and the second insulating film 19 of, say,silicon oxide. For this reason, the magnitude of an electric fieldapplied to the edges of the gate electrode during programming or erasingis reduced due to the effect of fringe capacitance. However, the effectof the fringe capacitance can be reduced by increasing the dimension ofthe first electrode layer 16-1 in the direction of gate length and themagnitude of the electric field applied to the gate insulating film ofthe memory cell transistor can be increased. For this reason, it becomespossible to prevent the write/erase characteristics of the memory celltransistor from being degraded.

FIG. 3 shows the circuit arrangement of a NAND nonvolatile semiconductormemory as an example of a nonvolatile semiconductor storage device usingmemory cells of the structure shown in FIGS. 1 and 2. FIG. 4 is aschematic plan view of the NAND nonvolatile semiconductor memory of FIG.3.

As shown in FIG. 3, the semiconductor storage device, indicatedgenerally at 70, is provided with a number of NAND cell units. Aplurality of NAND cell units forms a memory cell block, and a pluralityof memory cell blocks constitutes a memory cell array. Here, thesemiconductor storage device 70 is a NAND flash memory.

The NAND cell unit is provided with select transistors STR on the sideof each bit line BL connected to a sense amplifier (not shown) and onthe side of a source line SL. Between the two select transistors STRassociated with the same bit line are connected in series a plurality ofmemory cell transistors MTR. Each of bit lines BL1, BL2 and BL3intersects a control line SGD, word lines WL1, WL2, . . . , WLn, acontrol line SGS, and the source line SL.

The control line SGD is connected in common to the gates of the selecttransistors STR on the sense amplifier side of the bit lines BL1 to BL3.The word line WLn is connected in common to the control gates of memorycell transistors MTR each of which is the n-th one of the celltransistors in series connected to a respective one of the bit lines BL1to BL3. The word line WL4 is connected in common to the control gates ofmemory cell transistors MTR each of which is the fourth one of the celltransistors connected in series with a respective one of the bit linesBL1 to BL3. The word line WL3 is connected in common to the controlgates of memory cell transistors MTR each of which is the third one ofthe cell transistors connected in series with a respective one of thebit lines BL1 to BL3. The word line WL2 is connected in common to thecontrol gates of memory cell transistors MTR each of which is the secondone of the cell transistors connected in series with a respective one ofthe bit lines BL1 to BL3. The word line WL1 is connected in common tothe control gates of memory cell transistors MTR each of which is thefirst one of the cell transistors connected in series with a respectiveone of the bit lines BL1 to BL3. The control line SGS is connected incommon to the gates of the select transistors STR on the source lineside of the bit lines BL1 to BL3.

In the semiconductor storage device 70, as shown in FIG. 4, the sourceline SL, the control line SGS, the word lines WL1 to WLn and the controlline SGGD are arranged apart from one another and in parallel with oneanother in the horizontal direction of the drawing. The bit lines BL1 toBL3 are arranged apart from one another and in parallel with one anotherin the vertical direction of the drawing. The device regions are formedbelow each of the bit lines BL and the device isolation regions isformed between each device region.

That is, it can be said that the semiconductor substrate is separatedinto a number of device regions by the device isolation regions. At theintersections of the source line SL and the bit lines BL1 to BL3, sourceline contacts SLC are formed. A bit line contact BLC is formed in thatportion of each bit line BL which is located between the control lineSGD and the corresponding sense amplifier (not shown).

At each of the intersections of the word lines WL1 to WLn and the bitlines BL1 to BL3, one of the memory cell transistors MTR is placed.Likewise, at each of the intersections of the control lines SGS and SGDand the bit lines BL1 to BL3, one of the select transistors STR isplaced.

A sectional view taken along line A-A of FIG. 4 corresponds to FIG. 1,while a sectional view taken along line B-B of FIG. 4 corresponds toFIG. 2.

Reference is next made to FIGS. 5A through 5F to describe a method ofmanufacturing the nonvolatile semiconductor memory according to thepresent embodiment.

First, as shown in FIG. 5A, a film of silicon oxide is formed on thesilicon substrate 11, the film of silicon oxide serves as the tunnelinsulating film 12 of the memory cell transistors, at a thickness of,say, 4 nm through, for example, thermal oxidation. Next, well/channelregions (not shown) of the memory cells are formed by ion implantation.Then, a film of silicon nitride is formed the tunnel insulating film 12,the film of silicon nitride serves as the charge storage layer 13, at athickness of 7 nm through the use of, for example, chemical vapordeposition (CVD).

Next, though not shown, a mask material consisting of stacked films of,for example, silicon oxide and silicon nitride is deposited onto thecharge storage layer 13 and then predestinate areas of device isolationregions are opened by lithography. The mask material, the charge storagelayer 13, the tunnel insulating film 12 and the silicon substrate 11 areetched in sequence to form trenches for device isolation regions in thesilicon substrate 11. After that, the trenches formed in the siliconsubstrate 11 are filled with a device isolation insulating film of, forexample, silicon oxide. Subsequently, the device isolation insulatingfilm is planarized by chemical mechanical polishing (CMP) and thenetched so that its surface is at substantially the same level as the topof the charge storage layer 13. After that, the mask material isremoved. Thereby, the device isolation regions 14 are formed.

As shown in FIG. 5B, a film of Al₂O₃ that forms the block insulatingfilm 15 is formed to a thickness of, say, 15 nm. After that, a TaN filmof 10 nm thickness that forms the first gate electrode layer 16-1, apolysilicon film of 40 nm thickness that forms the second electrodelayer 16-2 and a silicon nitride film 21 which forms the mask materialfor gate electrode processing are formed in sequence. Here, the TaN filmhas a large work function and is particularly useful as a material whichdoes not react with Al₂O₃.

As shown in FIG. 5C, the silicon nitride film 21 is etched into the gateelectrode pattern by lithographic techniques and then the second gateelectrode layer 16-2 is selectively etched by reactive ion etching (RIE)using the etched silicon nitride film 21 as a mask.

As shown in FIG. 5D, the silicon oxide film 17 is deposited to form asidewall mask for processing the first gate electrode layer 16-1 andthen a sidewall insulating film (first insulating film) is formed byetching. Specifically, the silicon oxide film is deposited over theentire surface in the state shown in FIG. 5C. Then a sidewall insulatingfilm is etched by an anisotropic etching, such as RIE, to leave thesilicon oxide film 17 only on the sidewalls of the second gate electrodelayer 16-2. That is, a sidewall insulating film consisting of thesilicon oxide film 17 is formed in a self-aligned manner.

As shown in FIG. 5E, using the silicon nitride film 21 and the siliconoxide film 17 as a mask, the first gate electrode layer (TaN) 16-1, theblock insulating film (Al₂O₃) 15, the charge storage layer (siliconnitride) 13 and the tunnel insulating film (silicon oxide) 12 are etchedin sequence. Thereby, each of the films from the first gate electrodelayer 16-1 through the tunnel insulating film 12 is processed to have alarger dimension in the direction of gate length than the second gateelectrode layer 16-2 by twice the thickness (dimension in the directionof gate length) of the sidewall insulating film 17.

In this etching process, etching may be stopped after the first gateelectrode layer 16-1 (or the first gate electrode layer 16-1 and theblock insulating film 15, or the first gate electrode layer 16-1, theblock insulating film 15 and the charge storage layer 13) has beenselectively etched away. This is, adjacent memory cells can operate asMONOS cells if at least the first gate electrode layer 16-1 has beenseparated into adjacent gate electrodes of the respective memory cells.

As shown in FIG. 5F, the source/drain diffusion layers 18 of the memorycell transistors are formed in the surface of the silicon substrate 11by ion implantation using the silicon nitride film 21 and the siliconoxide film 17 as a mask. Then, the silicon oxide film (second insulatingfilm) 19 is deposited and then a top of the silicon oxide film isplanarized by CMP.

Next, the silicon nitride film 21 are etched away to expose the topsurface of the second gate electrode layer 16-2. At that time, thesilicon oxide film 17 and the silicon oxide film 19 may be partly etchedaway and side surface of top of the second gate electrode layer 16-2 isexposed. Then, a top portion of the second gate electrode layer 16-2 isturn into a silicide. A film of NiSi, which forms the third gateelectrode layer 16-3 of low resistivity, is then formed to a thicknessof, say, 20 nm, thereby obtaining the structure shown in FIGS. 1 and 2.Though not shown, this process causes the upper portion of the thirdgate electrode layer 16-3 to expand in the direction of gate length.

After that, an interlayer insulating film, contact electrodes and layersof interconnections are formed using generally known techniques, wherebya nonvolatile semiconductor memory is completed.

According to the present embodiment, as described above, the secondelectrode layer 16-2 and the third electrode layer 16-3 are formed withthe insulating film 17 on the side, but the first electrode layer 16-1is not formed with the insulating film 17 on the side. Then it ispossible to prevent the gate length from being reduced while suppressingshort-circuiting of adjacent gates. Since the spacing between adjacentgates is short, it is possible to secure sufficient gate length even ifthe edges of the first electrode layer 16-1 are oxidized.

Accordingly, the deterioration of the characteristics of memory celltransistors, such as the deterioration of the short-channelcharacteristic, the deterioration of the program/erase characteristicdue to the effect of fringe capacitance at the edges of the gateelectrodes, etc., can be prevented.

In addition, the use of an insulating film which has a high dielectricconstant like Al2O3 as the block insulating film 15 allows its leakagecurrent to be reduced. Moreover, the use of a material which contains ametal, such as TaN, and has a large work function as the first gateelectrode layer 16-1 allows the work function of the gate electrode tobe made large. Thereby, it becomes possible to suppress the injection ofelectrons from the gate electrode through the block insulating film intothe charge storage film in an erase operation and prevent thedeterioration of the erase characteristic of the memory celltransistors. Furthermore, the use of a silicide as the third gateelectrode 16-3 allows the gate electrode 16 to have low resistivity.

Second Embodiment

FIG. 6 is a sectional view in the direction of gate length of a memorycell transistor portion of a NAND nonvolatile semiconductor memoryaccording to a second embodiment.

The second embodiment is different from the first embodiment in that thefirst gate electrode layer 16-1 contains oxygen in its edge portionswhich are opposed to each other in the direction of gate length.

As shown in FIG. 6, the first gate electrode layer 16-1 is formed from afilm 16-1-1 of TaN in its central portion and a layer 15-1-2 whichcontains oxygen in TaN in its edge portions.

With the structure shown in FIG. 6, the layer 16-1-2 which containsoxygen in TaN, while having a larger resistivity than the TaN film16-1-1, functions as a gate electrode; thus, the gate length in notshort because of the film 16-1-2 will not reduce. Moreover, that theresistivity of the layer 16-1-2 which contains oxygen in TaN is largerthan that of the TaN film 16-1-1 is effective in increasing thebreakdown voltage between adjacent gate electrodes.

In addition, breakdown may occur when a high electric field is appliedto the edges of the block insulating film (Al2O3) film 15, the chargestorage layer (silicon nitride) 13 and the tunnel insulating film(silicon oxide) 12 which have reliability lowered due to etching damagesof the gate electrodes forming. In the case of the present embodiment,however, the gate electrode layer 16-1 is made lower in resistivity inthe edge portions than in the central portion owing to introduction ofoxygen into the edge portions; thus, it becomes possible to preventapplying high electric field to the edges of the above-mentionedinsulating films. Thereby, the reliability of the memory celltransistors can be increased.

The structure of the present embodiment can be fabricated in thefollowing manner:

The structure shown in FIG. 5E is fabricated in the same manner as inthe case of the first embodiment described previously. After that, heattreatment is carried out on the structure in an oxygen atmosphere,whereby the layer 16-1-2 which contains oxygen in TaN is formed at thoseedges of the first gate electrode layer 16-1 which are opposed to eachother in the direction of gate length.

Even if the structure shown in FIG. 5F is heat treated without oxygen,oxygen can be supplied from the silicon oxide film 19 to form the layer16-1-2 which contains oxygen in TaN at the edges of the first gateelectrode layer 16-1 which are opposed to each other in the direction ofgate length.

In order to reduce the number of processing steps, the heat treatmentcan also be carried out simultaneously with annealing to immobilizeimpurities in the source/drain diffusion layers 18. For example, afterthe structure shown in FIG. 5E has been fabricated, the gate electrode16 is formed with a thin film of silicon oxide on the side of the gateelectrode 16 such an extent that it does not fill in the gap betweenadjacent gate electrodes. Then, using the silicon oxide film as a spacerfor implantation, the source/drain diffusion layers 18 of the memorycell transistors are formed in the surface of the silicon substrate 11through ion implantation. Next, heat treatment is carried out under nooxygen atmosphere, whereby oxygen is supplied from the thin film ofsilicon oxide to the first gate electrode layer 16-1 and the impuritiesin the source/drain diffusion layers 18 are immobilized. After that, thesilicon oxide film 19 is deposited. The subsequent processing stepsremain unchanged from the first embodiment.

The present embodiment can offer the same advantages as described aboveeven if the TaN film 16-1-1 is smaller than the second gate electrodelayer 16-2 in the dimension in the direction of gate length as shown inFIG. 7. In other words, the present embodiment can achieve the sameadvantage as described above, even if the width of the layer 16-1-2containing oxygen in TaN is smaller (FIG. 6) or larger (FIG. 7) than thewidth of the first insulating film 17.

In either of FIGS. 6 and 7, the dimension in the direction of gatelength of the first electrode layer 16-1 is larger than that of thesecond and third electrode layers 16-2 and 16-3, which allows thatdimension of the first electrode layer 16-1 function as the gateelectrode. Then the transistor gate length to be made large whilesuppressing short-circuiting of adjacent second or third electrodelayers 16-2 or 16-3. For this reason, it is possible to prevent thedeterioration of the characteristics of the memory cell transistors dueto the short-channel effect.

In addition, by making the dimension in the direction of gate length ofthe first electrode layer 16-1 large, the effect of fringe capacitancecan be reduced and consequently the magnitude of the electric fieldapplied to the gate insulating film of the memory cell transistors canbe increased, thus making it possible to prevent the program/erasecharacteristics of the memory cell transistors from being deteriorated.Moreover, the gate insulating film and the insulating film between gateelectrodes can be modified by the heat treatment carried out insupplying oxygen, thus preventing the reliability of the memory celltransistors from being lowered.

Modification

The present invention is not limited to the embodiments described above.In the embodiments, Al2O3 is used as the block insulating film; however,this is not restrictive. Use may be made of metal oxide films, such asHfSiOx, HfAlOx, LaAlOx, etc., which have a high dielectric constant. Inaddition, the first electrode layer of the gate electrode is not limitedto TaN, but use may be made of a dielectric material, such as TaC, whichdoes not react with the block insulating film and has a large workfunction. Moreover, the second electrode layer is not limited topolysilicon or NiSi, but use may be made of various metal materialswhich have low resistivity.

The thickness of each element need not be limited as illustrated in theembodiments but may be modified suitably according to specifications.Further, the memory cell transistor of the structure shown in FIGS. 1and 2 is not limited to the use in a NAND nonvolatile semiconductormemory but may be used in various nonvolatile semiconductor storagedevices.

The first gate electrode layer need not necessarily be etched verticallyat its edges, but may be tapered at the edges on the upper side.Likewise, the second gate electrode layer also need not necessarily haveits sidewall etched vertically, but may be tapered at the edges suchthat the upper side is retracted relative to the underside. However, inorder to suppress the short-circuiting of adjacent gates, it is requiredthat the dimension in the direction of gate length of the underside ofthe second gate electrode layer be smaller than the dimension in thedirection of gate length of the upper side of the first gate electrodelayer.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1-19. (canceled) 20: A nonvolatile semiconductor storage deviceincluding memory cells, the memory cells comprising: a semiconductorsubstrate; a tunnel insulating film formed above the semiconductorsubstrate; a charge storage layer formed above the tunnel insulatingfilm; an insulating film formed above the charge storage layer; ametal-containing layer formed above and in contact with the insulatingfilm, the metal-containing layer being processed into a gate pattern andincluding tantalum and oxygen at its edges; and a gate electrode layerformed above the metal-containing layer and constituted of a materialdifferent from that of the metal-containing layer, the gate electrodelayer having a dimension smaller in the direction of gate length thanthe metal-containing layer. 21: The device according to claim 20,wherein a material of the gate electrode layer is a metal. 22: Thedevice according to claim 20, wherein a first length in the direction ofgate length of an under surface of the gate electrode layer is smallerthan a second length in the direction of gate length of an under surfaceof the metal-containing layer. 23: The device according to claim 20,wherein the insulating film has a larger dielectric constant than thetunnel insulating film. 24: The device according to claim 20, wherein amaterial of the insulating film has a larger dielectric constant thansilicon oxide. 25: The device according to claim 20, wherein theinsulating film includes a metal oxide. 26: The device according toclaim 20, wherein the insulating film includes HfSiO. 27: The deviceaccording to claim 20, further comprising an intermediate layer formedbetween the metal-containing layer and the gate electrode layer, aresistivity of the gate electrode layer being lower than a resistivityof the intermediate layer. 28: The device according to claim 27, whereinboth sides of the gate electrode layer are located inside both sides ofthe metal-containing layer in the direction of gate length. 29: Thedevice according to claim 27, wherein a first length in the direction ofgate length of an under surface of the gate electrode layer is smallerthan a second length in the direction of gate length of an upper surfaceof the metal-containing layer and a third length in the direction ofgate length of an under surface of the metal-containing layer. 30: Thedevice according to claim 20, wherein a thickness of the gate electrodelayer is larger than a thickness of the metal-containing layer. 31: Anonvolatile semiconductor storage device including memory cells, thememory cells comprising: a semiconductor substrate; a tunnel insulatingfilm formed above the semiconductor substrate; a charge storage layerformed above the tunnel insulating film; an insulating film formed abovethe charge storage layer; a metal-containing layer formed above and incontact with the insulating film, the metal-containing layer beingprocessed into a gate pattern and including tantalum; and a gateelectrode layer formed above the metal-containing layer and constitutedof a material different from that of the metal-containing layer, thegate electrode layer having a dimension smaller in the direction of gatelength than the metal-containing layer. 32: The device according toclaim 31, wherein a material of the gate electrode layer is a metal. 33:The device according to claim 31, wherein a first length in thedirection of gate length of an under surface of the gate electrode layeris smaller than a second length in the direction of gate length of anunder surface of the metal-containing layer. 34: The device according toclaim 31, wherein the insulating film has a larger dielectric constantthan the tunnel insulating film. 35: The device according to claim 31,wherein a material of the insulating film has a larger dielectricconstant than silicon oxide. 36: The device according to claim 31,wherein the insulating film includes a metal oxide. 37: The deviceaccording to claim 31, wherein the insulating film includes HfSiO. 38:The device according to claim 31, further comprising an intermediatelayer formed between the metal-containing layer and the gate electrodelayer, a resistivity of the gate electrode layer being lower than aresistivity of the intermediate layer. 39: The device according to claim31, wherein a thickness of the gate electrode layer is larger than athickness of the metal-containing layer.